1. Field of the Invention.
The present invention relates to a transmitter providing an output representative of a process variable.
2. Description of the Prior Art.
Transmitters which sense a process variable or parameter and provide a signal representative of the sensed parameter find widespread use in industrial process control systems. The transmitter is typically connected by two, three or four wires to a power source and a load. A two-wire transmitter includes a pair of terminals which are connected in a current loop with the power source and load so that the transmitter is powered by the loop current, while varying the magnitude of the loop current as a function of the sensed parameter. Three-wire and four-wire transmitters have separate leads for supply (energization) current and an output such as a current representing the process variable.
Electromechanical and analog electrical transmitters for sensing parameters such as pressure and temperature have been used for many years. More recently, with the advent of low power integrated circuits and low cost computer technology, transmitters have been developed which include some digital circuitry and, in some cases, microcomputers.